Answers

They actually all do. Even with striking you can use your opponents strength and then it takes very little effort but the striking is so much more powerful. Unfortunately very few teachers teach striking this way anymore since it is not used in sports. Since it is a more advanced way of striking and requires picking targets, setting up openings and really good timing to do this most students do not get to this level. Some are just satisfied to learn how to hit hard since that too is effective and put no effort into getting better.
Arts like Aikido, Aikijutsu, Judo, Jujitsu, Taijitsu and Taiji (Taichi) teach this concept much earlier with the thought that it takes so long to develop this skill that an early start will give you more time to develop it.